ITO thin films used to form transparent conductive films are widely used as transparent electrodes for display devices, particularly, liquid crystal displays. ITO thin films are often formed by sputtering techniques. However, when an ITO target is used for sputtering, a large number of black protrusions of several .mu.m to several mm size appear in an eroded part of a surface of the target. These protrusions are considered to be low-level oxides of indium and are commonly called nodules.
As the amount of sputtering time increases, the number and size of nodules increase and they gradually spread over the target surface. Conductivity of the nodules are lower than that of other portions, and if a large amount of nodules are produced, abnormal discharge (arcing) may often occur to produce particles on the ITO film or to preclude sputtering from being continued.
The production of nodules is unique to the ITO sputtering target used to form a transparent conductive film. If the number of nodules on the target surface increases abnormally, the sputtering operation is halted to execute reproduction processing in which the nodules produced on the target surface are scraped off. The halting of sputtering, however, may significantly degrade the productivity of continuous operations. Despite its simplicity, the reproduction operation requires appropriate skills and an appropriate amount of time and is thus cumbersome.
In addition, the reproduction processing for scraping the nodules off has limited results, and it is actually difficult to reproduce the ITO sputtering target to the extent that it is equivalent to a newly created target. Thus, the target may frequently become unavailable due to an increase in the number of nodules before it is fully used (i.e., before a target is consumed in a normal state). Consequently, to improve productivity while reducing material costs, an ITO sputtering target which produces a lesser amount of nodules is required.
The ITO sputtering target is generally obtained by grinding a sintered compact using a lathe or the like. Therefore, one of the causes of the production of nodules is assumed to be ITO ground powders which become deposited on the target surface. Accordingly, the amount of nodules produced is expected to be decreased by reducing the amount of ITO ground powders on the target surface. It has been difficult, however, to determine the threshold of the amount of ITO ground powders at which the nodule reduction effect starts to work and to reduce the amount of ITO ground powders to this threshold during actual operations.
Consequently, a polishing technique using a wet rotary polishing machine has been proposed to experimentally reduce the amount of ITO ground powders deposited on the target surface (Japanese Patent Application Laid-Open No. 8-60352), or a sandblast has been used to reduce the amount of ground powders (Japanese Patent Application Laid-Open No. 9-104973).
These methods, however, can reduce the amount of ground powders to some degree, but are still insufficient due to a large amount of ITO ground chips left on the surface. Therefore, since the threshold of the amount of ground powders is unknown, it is more appropriate to believe that the amount of ground powders deposited on the target surface has not actually been proved to be one of the causes of the production of nodules.